Add the ability to decode a subset to SkCodec.

dm/DM.cpp

Index: dm/DM.cpp
diff --git a/dm/DM.cpp b/dm/DM.cpp
index dd072dcec6d86fbf87fa1970175b5ce645f79024..86146a89354cb604de04b219a65bcf2715594b85 100644
--- a/dm/DM.cpp
+++ b/dm/DM.cpp
@@ -257,6 +257,8 @@ static void push_codec_srcs(Path path) {
                 CodecSrc::kGetFromCanvas_DstColorType, scale));
         push_src("image", "stripe", new CodecSrc(path, CodecSrc::kStripe_Mode,
                 CodecSrc::kGetFromCanvas_DstColorType, scale));
+        push_src("image", "codec_subset", new CodecSrc(path, CodecSrc::kSubset_Mode,
+                CodecSrc::kGetFromCanvas_DstColorType, scale));
     }
 }